Glass overglaze inks are used to protect printed circuit board elements and composites. Such overglaze inks must be able to form high density films compatible with the printed circuit board substrate; must be able to be fired in air and/or nitrogen to a temperature of about 600.degree.-900.degree. C.; must be chemically resistant to acidic or basic environments; must be capable of providing good hermeticity to the cofired substrate having elements thereon; and must be able to withstand repeated temperature cycling from room temperature to about 400.degree. C without cracking or peeling. Further, the fired overglaze film must have thermal expansion characteristics similar to the printed circuit board substrate.
In copending application Ser. No. 07/455,113 filed Dec. 22, 1989 entitled "DEVITRIFYING GLASSES AND DIELECTRIC INKS MADE THEREFROM", glass compositions suitable for high density dielectric inks were disclosed. The glasses in these inks contain from about 15 to about 23% by weight of zinc oxide; from about 7 to about 11% by weight of magnesium oxide; from about 14 to about 17% by weight of barium oxide; from about 9 to about 16% by weight of aluminum oxide; from 0 to about 2% by weight of phosphorus pentoxide; from 0 to about 3% by weight of zirconium silicate; and from about 35 to about 50% by weight of silicon oxide. These glasses are useful in making printed circuit boards in which the substrates are silicon because the thermal coefficients of expansion of the glasses are closely matched to that of silicon. However, it is also desirable to employ alumina as a printed circuit board substrate. Alumina has a higher coefficient of thermal expansion than silicon, and the above glasses are not thermally matched to alumina substrates. When an overglaze glass has a thermal coefficient of expansion that is lower than the substrate, the substrate will bend upon firing and become distorted.
Thus a devitrifying glass that can form an ink having good overglaze properties and that is thermally matched to alumina substrates has been sought.